1. Field of the Invention
The present invention relates to an electro-optical apparatus, a driving substrate for use in an electro-optical apparatus and a method of manufacturing them. Particularly, it relates to a structure and a method suitable, for example, to a liquid crystal display device comprising a dual gate type film insulated gate field effect transistor (hereinafter referred to as dual gate MOSTFT) using a single crystal silicon layer graphoepitaxially grown on an insulation substrate as an active region and a passive region.
2. Description of Related Art
Known active matrix type liquid crystal display devices include those having a display section using amorphous silicon for a TFT and an external driving circuit IC, or integration types comprising a display section and a driving circuit using polycrystal silicon formed by a solid phase growing method for a TFT (Japanese Published Unexamined Patent Application No. Hei 6-242433), and integration types comprising a display section and a driving circuit using polycrystal silicon subjected to excimer laser annealing for TFT (Japanese Published Unexamined Patent Application No. Hei 7-131030).
However, since the existent amorphous silicon TFT, although having good productivity, has electron mobility as low as about 0.5 to 1.0 cm2/v.sec, p-channel MOSTFT (hereinafter referred to as pMOSTFT) can not be prepared. Accordingly, since the peripheral driving section using pMOSTFT can not be formed on an identical glass substrate with that for the display section, a driver IC is attached externally and mounted by a TAB system, for example, so that it is difficult to reduce the cost. In addition, this leads to a limit for high fineness. Further, since the electron mobility is as low as about 0.5-1.0 cm2/v.sec, no sufficient ON current is available, and the size of the transistor is inevitably enlarged when it is used in the display section, which is disadvantageous for attaining a large aperture ratio of pixels.
Further, since the electron mobility of the existent polycrystal silicon TFT is 70 to 100 cm2/v.sec and can cope with high fineness, an LCD (Liquid Crystal Display Device) using a driving circuit integrated type polycrystal silicon TFT has been noted in recent years. However, in a case of a large LCD of 15 inches or more, since the electron mobility of polycrystal silicon is from 70 to 100 cm2/v.sec, the driving performance is insufficient and, after all, external driving circuit IC is necessary.
Further, since a TFT using a polycrystal silicon film formed by a solid phase growing method requires annealing at 600xc2x0 C. or higher for several hours and formation of gate SiO2 by thermal oxidation at about 1000xc2x0 C., semiconductor production equipment has to be adopted. Therefore, the size of wafers is limited to 8-12 inchxcfx86 and use of highly heat resistant and expensive quartz glass is inevitable making it difficult for the reduction of cost. Accordingly, application use is limited to EVF or data/AV projector.
Furthermore, the polycrystal silicon TFT obtained by the existent excimer laser annealing described above involves many problems in view of the power stability of the excimer laser, such as productivity, increase in the cost of the facility due to increased scale and lowering of yield/quality.
Particularly, in a large sized glass substrate, for example, of 1 m2, the problems become more significant to result in further difficulty for the improvement of the performance/quality and reduction of the cost.
It is, accordingly, an object of the present invention to manufacture an active matrix substrate incorporated with a high performance driver and an electro-optical apparatus such as a thin film semiconductor device for display use using the substrate by forming a film of a single crystal silicon layer of high electron/hole mobility at a relatively low temperature and uniformly, thereby enabling to obtain an integrated structure of a display section comprising n-channel MOSTFT (hereinafter referred to as nMOSTFT) of an LDD (Lightly Doped Drain) structure having high switching characteristics and low leak current or pMOSTFT or a complementary film insulated gate field effect transistor (hereinafter referred to as cMOSTFT) of high driving performance, and a peripheral driving circuit comprising cMOSTFT, nMOSTFT or pMOSTFT, or the combination thereof, attain a display panel of high quality, high fineness, narrow frame edge, high efficiency and large screen area, use even a large sized glass substrate with relatively low distortion point, provide high productivity, save the use of expensive production facility making it possible for cost reduction and, further, easy control for a threshold value and making it possible for high speed operation by the lowering of resistance and for large scaled screen.
The present invention provides an electro-optical apparatus having, on a first substrate (driving substrate here and hereinafter), a display section in which pixel electrodes (for example, a plurality of pixel electrodes disposed in a matrix) are arranged, and a peripheral driving circuit section disposed to the periphery of the display section and in which a predetermined optical material such as liquid crystal is interposed between the first substrate and a second substrate (opposing substrate here and hereinafter), and a driving substrate for use in the electro-optical apparatus, wherein
a gate portion comprising a gate electrode and a gate insulation film is formed on one surface of the first substrate,
a step is formed on one surface of the first substrate,
a single crystal silicon layer is formed on the first substrate including the step and the gate portion, and
a dual gate type first thin film transistor having the single crystal silicon layer as a channel region, a source region and a drain region, and having the gate portion above and below the channel region respectively constitutes at least a portion of the peripheral driving circuit section,
In the present invention, the thin film transistor includes a field effect transistor (FET) (including a MOS type and a junction type, both of which can be used) and a bipolar transistor, and the invention is applicable to any of the transistors.
The present invention also provides a method of effectively manufacturing an electro-optical apparatus having, on a first substrate, a display section in which pixel electrodes are arranged, and a peripheral driving circuit section disposed to the periphery of the display section and in which a predetermined optical material such as liquid crystal is interposed between the first substrate and a second substrate, and a driving substrate for use in the electro-optical apparatus, wherein the method comprises
a step of forming a gate portion comprising a gate electrode and a gate insulation film on one surface of the first substrate,
a step of forming a step on one surface of the first substrate,
a step of graphoepitaxially growing a single crystal silicon layer on the first substrate including the step and the gate portion using the step as a seed, for example, by a catalyst CVD process or a high density plasma CVD process,
a step of applying a predetermined treatment to the single crystal silicon layer to form a channel region, a source region and a drain region and
a step of forming a dual gate type first thin film transistor having the gate portions above and below the channel region respectively and constituting at least a portion of the peripheral driving circuit section.
According to the present invention, since a single crystal silicon layer is graphoepitaxially grown using the step formed on the substrate as a seed, for example, by a catalyst CVD process and a high density plasma CVD process, which is used, for example, for a dual gate type MOSTFT in a peripheral driving circuit of a driving substrate such as an active matrix substrate, or a dual gate type MOSTFT in a peripheral driving circuit of an electro-optical apparatus such as a display section-peripheral driving circuit integration type LCD, the following remarkable functions and effects (A)-(G) can be obtained.
(A) Since a step of a predetermined shape/size is formed on a substrate and a single crystal silicon layer is graphoepitaxially grown using the angle at the bottom of the step (bottom angle) as a seed, to obtain the single crystal layer of an electro mobility higher than 540 cm/v.sec, an electro-optical apparatus such as a thin film semiconductor device for display use incorporated with a high performance driver can be manufactured. In this case, the step is preferably formed as a concave portion such that the lateral side in cross section is orthogonal to the bottom face, or inclined toward the lower end at a bottom angle preferably of 90xc2x0 or less.
(B) Particularly, since the single crystal silicon layer shows high electron/hole mobility equal with that of a single crystal silicon substrate as compared with an existent amorphous silicon layer or polycrystal silicon layer, the single crystal silicon dual gate type MOSTFT obtained by the process can constitute an integrated structure having a display section comprising nMOS, pMOSTFT, or cMOSTFT having high switching characteristics (preferably with an LDD (Lightly Doped Drain) structure of moderating an electric field strength to lower the leak current) and a peripheral driving circuit section comprising cMOS, nMOS, pMOS TFT or a combination thereof of high driving performance, to provide a display panel of high image quality, high fineness, narrow frame edge, high efficiency and large screen. Particularly, it is difficult to form a pMOSTFT of high hole mobility for LCD TFT by using polycrystal silicon, but the single crystal silicon layer in the present invention shows a sufficiently high mobility also for positive holes, so that a peripheral driving circuit that drives electrons and holes either individually or in combination of them can be manufactured and can provide a panel integrating the same with TFT for display section of nMOS, pMOS or cMOS of LDD structure. Further, in a small-medium sized panel, it is possible to save one of a pair of peripheral vertical driving circuits.
(C) Particularly, since the dual gate type MOSTFT is used in the peripheral driving circuit, cMOS, nMOS or pMOS TFT having a driving performance 1.5 to 2 times as high as the single gate TFT can be constituted, to provide TFT of higher performance and larger driving performance, which is suitable particularly in a case of requiring a large driving performance TFT to a portion of the peripheral driving circuit. For example, it is considered that not only one of the pair of peripheral vertical driving circuits can be saved but also the present invention is advantageous when applied to organic EL or FET as the electro-optical apparatus other than LCD. Further, the dual gate structure is also advantageous in that it can be changed easily either to the top gate type or the bottom gate type by the selection of upper and lower gate portions and one of the upper and the lower gate portions can be used even when the other of them becomes not operable.
(D) Then, since the step is used as a seed for graphoepitaxial growing, and the single crystal silicon layer can be formed on the step by a cold film forming technique such as a catalyst CVD process (chemical vapor deposition using catalyst: at a substrate temperature of 200 to 800xc2x0 C., particularly, 300 to 400xc2x0 C.), a single crystal silicon layer can be formed uniformly at low temperature on the substrate. Accordingly, it is possible to use those substrates which are easily available at a reduced cost and having satisfactory physical properties such as glass substrates of relatively low distortion point or heat resistant organic substrates and, in addition, the size of the substrate can also be enlarged.
(E) Since annealing at a middle temperature for long time (about 600xc2x0 C., for ten and several hours) as in the case of the solid phase growing and excimer laser annealing are no more required, the productivity is high and no expensive production facilities are required, making it possible to reduce the cost.
(F) In the graphoepitaxial growing, since a single crystal silicon layer having a wide range of P- or N-conduction type and high mobility can be obtained easily by controlling the gas compositional ratio, the heating temperature for the substrate and the cooling rate in the catalyst CVD or the like, Vth (threshold value) can be controlled easily and high speed operation is possible by lowering the resistance.
(G) Further, when a group III or group V impurity element (for example, boron, phosphorus, antimony, arsenic, bismuth or aluminum) is doped in an appropriate amount separately from a doping gas upon forming the film of single crystal silicon by the catalyst CVD or the like, it is possible to optionally control the impurity species and/or concentration thereof, namely, the p-type/n-type conduction type and/or carrier concentration of the single crystal silicon layer.